1. Field of the Invention
This invention relates generally to a device for indicating the amount of hydrogen in a liquid hydrogen tank and, more particularly, to a device for indicating the amount of hydrogen in a liquid hydrogen storage tank, where the device employs inductively coupled coils inside and outside of the tank so as to limit the heat flow through the tank walls.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
In an automotive fuel cell application, the hydrogen is sometimes stored in a cryogenic tank on the vehicle, where the hydrogen is a liquid at very cold temperatures, such as 25° K. The cryogenic tank typically includes an inner tank and an outer tank with a vacuum and a multilayer insulation (MLI) layer therebetween to limit heat penetration into the inner tank to maintain the liquid hydrogen in its super cold state.
FIG. 1 is a cross-sectional plan view of a known liquid hydrogen tank assembly 10 of this type. The tank assembly 10 includes an inner tank 12, an outer tank 14 and a vacuum and MLI layer 16 therebetween. Liquid hydrogen 18 is stored Within the inner tank 12 in a cryogenic state. The liquid hydrogen 18 is removed from the inner tank 12 through a suitable mechanical cryo-valve (not shown). As the liquid hydrogen 18 is removed from the tank 12, the remaining volume within the inner tank 12 includes gaseous hydrogen 20.
It is desirable to know the level of the liquid hydrogen 18 within the inner tank 12 so that the tank assembly 10 can be refilled at the appropriate time. Therefore, the tank assembly 10 includes a level sensor 22 positioned within the inner tank 12 for this purpose. The level sensor 22 provides an electrical signal indicative of the level of the liquid hydrogen 18 within the tank 12 on output lines 24 that is received by a signal conditioning circuit 26 outside of the outer tank 14. An output signal from the signal conditioning circuit 26 provides the level of the liquid hydrogen 18 in the tank assembly 10.
FIG. 2 is a schematic diagram of the level sensor 22 separated from the tank assembly 10. The level sensor 22 includes an outer conductive tube 30 and an inner conductive tube 32 coaxially aligned so as to define a gap 34 therebetween. The conductive tubes 30 and 32 can be made of any conductive material suitable for a cryogenic environment, such as stainless steel. Space holders 36 maintain the gap 34 to allow the liquid hydrogen 18 to flow between the tubes 30 and 32.
The level sensor 22 operates on the principal that an electrical capacitance exists between the inner tube 32 and the outer tube 30, and this capacitance will be different when the gaseous hydrogen 20 is the dielectric and when the liquid hydrogen 18 is the dielectric. As the level of the liquid hydrogen 18 within the tank 12 changes, the capacitance between the inner tube 32 and the outer 30 also changes. Particularly, the liquid hydrogen 18 will be more conductive than the gaseous hydrogen 20 so that the capacitance of the level sensor 22 will decrease as the level of the liquid hydrogen 18 within the inner tank 12 decreases. The signal conditioning circuit 26 applies an AC signal to the tubes 30 and 32, and the capacitance of the level sensor 22 changes in response to the AC signal depending on the level of the liquid hydrogen 18 within the tank 12. The capacitive output from the lines 24 is calibrated to provide an indication of the amount of the liquid hydrogen 18 within the tank assembly 10.
When the fuel cell system is shut off, the liquid hydrogen 18 has a very low temperature. As time passes the temperature of the hydrogen 18 within the tank 12 slowly increases because of heat conductive paths from the tank 12 to the outside environment. As the temperature of the hydrogen 18 increases, the pressure within the tank 12 also increases. However, the pressure within the tank 12 is limited to a critical value, referred to as the boil-off pressure. If the pressure within the tank 12 reaches the boil-off pressure, hydrogen must be released from the tank assembly 10 to prevent a further increase of the pressure, which is undesirable. The time from when the fuel cell system is shut off to when the boil-off pressure is reached in the tank 12 is the autonomy time. Because vehicles are sometimes not operated for extended periods of time, it is desirable to maximize the autonomy time by minimizing the heat loss from the tank assembly 10.
Because the wires 24 extend through the walls of the inner tank 12, the vacuum and MLI layer 16 and the outer tank 14 to the outside environment, they provide a conductive heat path from the external environment to the liquid hydrogen 18 within the tank 12. This heat conductive heat path increases the rate that the liquid hydrogen 18 is heated, and thus increases the pressure within the inner tank 12, reducing the autonomy time. Also, passing the wires 24 through the tanks 12 and 14 provides difficult design requirements for sealing the tanks 12 and 14. It would be desirable to reduce or eliminate this conductive heat path to increase the autonomy time of the tank assembly 10.